Wind turbines comprise a tower and a nacelle placed on top of that tower, whereby the nacelle is equipped with a rotor which rotates due to the impact of wind. In the nacelle, the rotational movement of the rotor is used to generate electric power.
Large wind turbine towers often comprise a number of tower segments, e. g. made of steel, which when assembled together form the complete wind turbine tower. The assembly of large wind turbine towers—in particular under offshore conditions, but also onshore—consumes a lot of time, effort, and financial resources.
Thereby, lifting (or raising, which is used as a synonym for lifting throughout this description, as well as “raising device” is used as a synonym for “lifting device”) the tower in segments or as a whole to the assembly site, i.e. to a foundation on which the tower is to rest to be later equipped with the nacelle and the rotor, poses several problems. Firstly, the tower (or segments thereof) has to be firmly held by a suitable raising device. One such raising device is described in the European patent EP 2 402 278 B1. Secondly, the tower is subject to enormous vibrations induced by the wind coming from its side. Such side winds may have high velocities—as can be expected especially in areas in which wind turbines are operated.
The above-indicated vibrations are commonly called vortex induced vibrations (VIV) which can be the result of alternating vortex shedding around the tower in strong wind conditions. This shedding results in alternating pressure differences over the wind turbine tower's cross-section perpendicular to the wind direction. In case the shedding frequency is close to the natural frequency of the wind turbine tower, alternating shedding is regular and the amplitude is large enough compared to the structural damping of the tower, VIV can occur which could cause damage to the wind turbine tower or reduce its fatigue life.
VIV have been known for a long time and several measures have been found of how to reduce these vibrations on readily installed tubular structures such as high chimney stacks. An overview over such measures is given in Blevins, R. D.: “Flow-Induced Vibration”. Malabar 2001 2nd edition, in particular in Chapter 3.6 (pp. 77ff.). One approach is to dampen the structure by increasing the weight on its top. Other approaches refer to streamline the cross-section of the structure by measures on its outer surface. An overview over these measures is given on page 78. Amongst these streamlining measures, a prominent one is that of the use of so-called helical strakes. The use of such strakes was first published by U.S. Pat. No. 3,076,533 assigned to Scruton et al.
The use of such measures on wind turbine towers however basically forbids itself because they do not only affect the tower, but indirectly the behaviour of the wind turbine itself: they influence the behaviour of the rotor blades, which means they reduce the affectivity of the wind turbine, i.e. the power output of electric power at a given wind speed.